Since the cluster could be in any part of the hive, the number of mites under the cluster will always be higher than not under the cluster. The typical standard is to do the math to end up with the total number of mites dropped in 24 hours. Even if you are counting over two days, you just divide by two. The number is really not so much a number as a trend. I would consider over 50 a moderately high number but if it's staying steady I wouldn't worry too much. if the number is going up exponentially (and sometimes it does) then you have a big problem. If the number is going down, then things are really good (or they are all in the cells...). Sometimes a threshold is thrown out there. usually it's some number between about 50 and 100 for a 24 hour drop that people get concerned.

Since I've been on small cell and the numbers dropped to almost nothing I have stopped counting and stopped worrying about Varroa. It has been a non issue for me for quite some time.